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It is unclear how plants optimize their preference for different forms of inorganic nitrogen to enhance their survival and fitness in challenging environments. Using a hydroponics culture system, we monitored morphological, physiological, and molecular changes in Malus hupehensis seedlings when drought conditions (induced by 5% polyethylene glycol, or PEG) were combined with a low or normal supply of N (0.05 mM or 1 mM NH 4 NO 3 , respectively). We found here that PEG-induced drought stress negatively inhibited the growth of Malus hupehensis seedlings and resulted in higher NH 4 + /NO 3 − ratios in their roots and leaves. The net ammonium influx at the surface of fine roots rose dramatically under drought treatment, while that of nitrate was less changed. This was confirmed by higher absorption of stable 15 NH 4 + isotope than of 15 NO 3 − under drought conditions. Consistently, two ammonium transporter genes closely related to ammonium uptake ( AMT4;2 and AMT4;3 ) were notably up-regulated in response to drought stress, while transcript levels of most genes related to nitrate uptake/reduction and nitrogen assimilation decreased. Therefore, in acclimating to drought, Malus hupehensis plants exhibited relatively increased uptake of NH 4 + compared to NO 3 − . In addition, reducing N supply may alleviate the adverse affect of drought stress on the growth of Malus hupehensis . [ABSTRACT FROM AUTHOR]

Guzmania monostachia (Bromeliaceae) is a tropical epiphyte capable of up-regulating crassulacean acid metabolism (CAM) in its photosynthetic tissues in response to changing nutrient and water availability. Previous studies have shown that under drought there is a gradient of increasing CAM expression from the basal (youngest) to the apical (oldest) portion of the leaves, and additionally that nitrogen deficiency can further increase CAM intensity in the leaf apex of this bromeliad. The present study investigated the inter-relationships between nitrogen source (nitrate and/or ammonium) and water deficit in regulating CAM expression in G. monostachia leaves. The highest CAM activity was observed under ammonium nutrition in combination with water deficit. This was associated with enhanced activity of the key enzyme phosphoenolpyruvate carboxylase, elevated rates of ATP- and PPi-dependent proton transport at the vacuolar membrane in the presence of malate, and increased transcript levels of the vacuolar malate channel- encoding gene, ALMT. Water deficit was consistently associated with higher levels of total soluble sugars, which were maximal under ammonium nutrition, as were the activities of several antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase). Thus, ammonium nutrition, whilst associated with the highest degree of CAM induction in G. monostachia, also mitigates the effects of water deficit by osmotic adjustment and can limit oxidative damage in the leaves of this bromeliad under conditions that may be typical of its epiphytic habitat. [ABSTRACT FROM AUTHOR]

Microcystis aeruginosa causes harmful algal blooms in the Nakdong River of Korea. We studied the effect of different concentrations and ratios of ammonium (NH4+), nitrate (NO3-), and phosphate (PO4 3-) on growth of this species in BG-11 medium: each nutrient alone, NO3- : NH4 + ratio, the N : P ratio with fixed total N (TN), and the N : P ratio with fixed total P (TP). The single nutrient experiments indicated that M. aeruginosa had the highest growth rate at NH4 + and NO3- concentrations of 500 μM, and at a PO4 3- concentration of 5 μM. The NO3- : NH4+ ratio experiments showed that M. aeruginosa had the highest growth rate at a ratio of 1 : 1 when TN was 100 μM and 250 μM, and the lowest growth rate at a ratio of 1 : 1 when the TN was 500 μM. The N : P ratio with fixed TN experiments indicated that M. aeruginosa had the highest growth rates at 50 : 1, 20 : 1, and 100 : 1 ratios when the TN was 100, 250, and 500 μM, respectively. In contrast, the N : P ratio with fixed TP experiments showed that M. aeruginosa had the highest growth rates at 200 : 1 ratio at all tested TP concentrations. In conclusion, our results imply that the NO3- : NH4+ ratio and the PO4 3- concentration affect the early stage of growth of M. aeruginosa. In particular, our results suggest that the maximum growth of M. aeruginosa is not simply affected by the NO3- : NH4+ ratio and the N : P ratio, but is determined by the TN concentration if a certain minimum PO4 3- concentration is present. [ABSTRACT FROM AUTHOR]

Nitrate (NO $ _3^ - $) and ammonium (NH $ _4^+ $) are the predominant forms of nitrogen (N) available to plants in agricultural soils. Nitrate concentrations are generally ten times higher than those of NH $ _4^+ $ and this ratio is consistent across a wide range of soil types. The possible contribution of these small concentrations of NH $ _4^+ $ to the overall N budget of crop plants is often overlooked. In this study the importance of this for the growth and nitrogen budget of maize ( Zea mays L.) was investigated, using agriculturally relevant concentrations of NH $ _4^+ $. Maize inbred line B73 was grown hydroponically for 30 d at low (0.5 mM) and sufficient (2.5 mM) levels of NO $ _3^ - $. Ammonium was added at 0.05 mM and 0.25 mM to both levels of NO $ _3^ - $. At low NO $ _3^ - $ levels, addition of NH $ _4^+ $ was found to improve the growth of maize plants. This increased plant growth was accompanied by an increase in total N uptake, as well as total phosphorus, sulphur and other micronutrients in the shoot. Ammonium influx was higher than NO $ _3^ - $ influx for all the plants and decreased as the total N in the nutrient medium increased. This study shows that agriculturally relevant proportions of NH $ _4^+ $ supplied in addition to NO $ _3^ - $ can increase growth of maize. [ABSTRACT FROM AUTHOR]

Ammonium nutrition is toxic to many plants. Arabidopsis displays high intraspecific variability in ammonium tolerance (shoot biomass), and ammonium accumulation seems to be an important player in this variability.Plants are dependent on exogenous nitrogen (N) supply. Ammonium (NH4+), together with nitrate (NO3–), is one of the main nitrogenous compounds available in the soil. Paradoxically, although NH4+ assimilation requires less energy than that of NO3–, many plants display toxicity symptoms when grown with NH4+ as the sole N source. However, in addition to species-specific ammonium toxicity, intraspecific variability has also been shown. Thus, the aim of this work was to study the intraspecific ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions. Plants were grown with either 1mM NO3– or NH4+ as the N source, and several parameters related to ammonium tolerance and assimilation were determined. Overall, high variability was observed in A. thaliana shoot growth under both forms of N nutrition. From the parameters determined, tissue ammonium content was the one with the highest impact on shoot biomass, and interestingly this was also the case when N was supplied as NO3–. Enzymes of nitrogen assimilation did not have an impact on A. thaliana biomass variation, but the N source affected their activity. Glutamate dehydrogenase (GDH) aminating activity was, in general, higher in NH4+-fed plants. In contrast, GDH deaminating activity was higher in NO3–-fed plants, suggesting a differential role for this enzyme as a function of the N form supplied. Overall, NH4+ accumulation seems to be an important player in Arabidopsis natural variability in ammonium tolerance rather than the cell NH4+ assimilation capacity. [ABSTRACT FROM AUTHOR]